Control mechanism for helicopters with coaxial rotors



Oct. 25, 1949. H. -r. PENTECOST I 2,436,059

CONTROL MECHANISM FOR HELICOPTERS WITH COAXIAL ROTORS Filed Oct. 9, 19453 Sheets-Sheet 2 HORACE 'T. PENTECOST INVENTOR.

Oct 1949- H. T. PENTECOST 2,486,059

CONTROL MECHANISM FOR HELICOPTERS WITH COAXIAL ROTORS 3 Sheets-Sheet 3Filed Oct. 9, 1945 QQA HORACE T PENTECOST v IN V EN TOR. $75M PatentedOct. 25, 1949 CONTROL MECHANISM FOR HELICOPTERS WITH COAXIAL ROTORSHorace T. Pentecost, Benton, near Seattle, Wash.

Application October 9, 1945, Serial No. 621,254

4 Claims. (01. I'm-135.24)

This invention relates to a control mechanism' for helicopters withco-axial rotors and, more particularly. to an operating and controlmeans for counter-rotary, co-axially bladed helicopters.

It is a prime object of the invention to provide improved and simplifiedcontrol of helicopter operation and flight.

Existing helicopters, including those of the coaxial counter-rotarytype. are now flown by trained operators who have learned to coordinatetheir movements of several different control elements to produce theproper collective adjustments which result in desired flight maneuvers.These individual control elements usually consist of: First, means forpositioning the angle of pitch of all blades collectively in order tochange the total lift of the rotors; second, means for obtainingcyclical blade angle pitching in order to create or change thehorizontal thrust necessary for lateral flight of the craft in anydirection; and, third, means for maintaining or rotating the draft inany desired direction about its upright axis while in flight.

A lever operable by hand is usually provided to adjust the pitch angleof all blades collectively, and by means of a twisting handgrip throttlemounted thereon engine power is also controlled. A second lever operatedby the pilot's other hand and free to be positioned in the direction ofdesired travel usually controls the cyclic pitching of the rotor blades.The usual method to control the direction of the craft about its uprightaxis includes right and left foot pedal elements which are selectivelydepressed by the operator depending upon the direction in which hedesires the craft to rotate.

It is evident from the above that the pilot thus engaged to maintainsatisfactory flight is physically fully occupied, since both his handsand both his. feet are continually required to effect controladjustments. Such other adjustments as may benecessary to engine orauxiliary equipment controls must then be made with difllculty and atthe expense of continuous safe flying control.

Other objects of the invention, therefore, are to impress collectivepitch and cyclic pitch upon the blades of a counter-rotary co-axialblade type helicopter by means of a single operator controlled element;to impress collective pitch variably upon the blades; to impress cyclicpitch upon the blades while at the same time and by the same meansvarying the collective pitch between the blades; and to impress bothcyclic and Gil collective pitch by a single manually operable means thatis also capable of varying the collective pitch of the blades.

Still other and important objects of the inventi on include theprovision of novel means for collectively pitching a pair of blades inhelicopters of the type mentioned above, and novel means for cyclicallypitching such blades, which means may or may not be associated with asingle control element.

Other objects and advantages of the invention will be apparent from thefollowing descriptions, the accompanying drawings and the appendedclaims.

In the drawings:

Figure 1 is a side elevation of a small helicopter embodying myimprovements;

Figure 2 is a schematic diagram of the transmission thereof:

Figure 3 is a perspective view of the rotor blade control means thereof,with various small sections broken away for convenience of illustration;

Figure 4 is a schematic view of the mechanism in the zero pitchposition;

Figure 5 is a schematic view of the mechanism in the full collectivepitch position;

Figure 6 is a schematic view of the mechanism in the forward cyclicpitch position;

Figure 7 is a schematic view of the mechanism in the transverse cyclicpitch position;

Figure 8 is a schematic view of the mechanism in the variable collectivepitch position.

Referring in detail to the drawing in which one form of the invention isillustrated, the aircraft comprises a frame of tubular members l0.adapted to support the pilot in suitable flying position. Three radiallyspaced members l2, ll, l5 form the ground engaging elements of frame inand serve to maintain lateral stability of the craft when at rest. Atube i8 is attached to the forward engaging element l2 and furnishes afoot rest for the pilot. Two canvas pieces 20 and 22 are attached to,and suspended between the tubular members of frame l0, furnishing a seatand back support for the pilot. A safety belt 24 and a manual throttlecontrol 26 are also mounted on frame it) at convenient locations.

-Also comprising the aircraft and rigidly attached to frame I is anengine 28, including a carburetor l0, spark plugs 32, exhaust stacks 3|,air-cooling bame 3'8, fuel line 38. and a fuel tank 40. One form ofengine which may be used to furnish power for sustaining flight of thecraft is of the internal combustion two stroke. two

cylinder opposed type. It is forced air-cooled by means of paddlesattached to the. flywheel which also houses the magneto. Starting'may beby means of a rope, knotted at one end and wrapped around the flywheelin the same manner as is customary with outboard-type marine motors.

I The flywheel is preferably mounted on the underside of the engine withthe crankshaft axis upright and the power end of the shaft at the top ofthe engine. Throttle 26 may be attached directly to carburetor 30 formanual control or an inertia type throttle governing device may be usedto maintain constant engine speed with manual throttle 26 serving as anover-ride control for use only when emergency power is required or when.

unusual operating requirements dictate its use. A switch button 42shorts the primary circuitof the engine magneto system and may beconveniently located on the flight control element where the operatormay quickly stop the engine by depressing the button with his thumb.

Also comprising the aircraft and fitted to the upper end of the engine28 is a transmission 44. The function of the transmission, more fullydescribed in my co-pending application, Serial No. 622,727, fliedOctober 17, 1945, now Patent No. 2,261,348, Is to furnish means forconverting and dividing engine torque between two coaxial counter-rotating rotors. as well as furnishing (a) means for manuallydisengaging all engine power and rotation from being transmitted to therotors and (b) means for automatically limiting the transmission oftorque through the system to the single or one way route from the enginepower end to the rotor shafts 66 and 10.

Figure 2 illustrates a method for obtaining the foregoing desiredcondition. A splined engine power-shaft 46 is fitted into a, cone clutch48 controlled manually by a clutch lever 80. A pinion 62, receivingpower through the clutch 48, and

a gear 84 reduce engine speed and angularly change the axis of rotation.A ratchet type overrun clutch 68, between gear 54 and pinion 62, allowsthe rotors to become automatically free to rotate in their normaldirections upon engine stoppage while still being interconnected bydifferential gears 58 and 60 through pinion 82. The upper rotor hub 64is connected, by means of the upper rotor shaft 66, to gear 58 and thelower rotor hub 88 is connected to gear 60 by means of the lower rotorshaft 10, these shafts being coaxial.

Attached to each rotor hub 84 and 68, and also comprising the aircraft,are pairs of rotor blades 12. These may be of the size and configurationmost advantageous in obtaining maximum performance and greatest economyfor the craft as determined by aerodynamic and power calculations. Ablade may be selected with a symmetrical airfoil section and a center ofpressure which falls approximately at the quarter chord point. A bladeis hinged to its rotor hub so that it may rotate about a longitudinalaxis coinciding with its center of pressure axis or at a point quarterdistant between leading and trailing edge. Very little force then isrequired to change its relative angle of attack throughout asufllciently wide range to encompass both the maximum and minimum anglesnormal to helicopter rotor blade operation in flight. For this reason,the pilot may be able to directly and manually control the blade anglesduring flight without undue effort.

The mechanism required to position and mamtain the blade anglesnecessary for controlled flight maneuvers then comprises and completesthe aircraft selected to illustrate my invention. In Figure 3, a controlhandle 14 may be moved by the pilot in any direction he wishes the craftto fly. It may be moved up or down as shown in Figures 4 and 5, theresult being a simultaneous increase or decrease in the ansle of attackor pitch of all rotor blades. With an upward push of the control handleand an increase of blade angle,'together with an increase in enginepower either by manual or automatic adjustment of the throttle control,the craft will climb. The upward force on the control handle I4 istransmitted by rod 16 through slide bushing I8 radially movable aboutpin 82. The latter pin is pivotally mounted stationary with the case oftransmission 44 bybifurcated arm 84. The fork yoke 86, rigidly attachedto rod 16, then forces two pins 88 and 80 down, which are connected toand in turn carry downward the lower gimbals assembly 82, composed of aninner ring 84 and track 88 and an outer ring 88 and track I00, a lowergimbals spacer yoke I02, and parts attached thereto. An outer ring shoeI04, engaged in the outer ring track I00, has rods I06 universallyattached thereto. Rods I06 are also universally connected to the uppergimbals assembly I08, composed of the upper ring H0 and track II2 andthe upper gimbals spacer yoke H4, and parts attached thereto. The latteris also forced downward upon an upward movement of control handle I4.

A blade pitch rod I I6 is universally attached at Q one end to thetrailing portion of a blade I2 and at the other end to a rotary trackshoe III, which in turn rides in a ring track 88, I I2. When the controlhandle 14 moves upward the trailing edge of all blades will be forceddown, and the result will be a simultaneous increase in all the anglesof attack of the blades. This will be true for any rotational speed ofthe counter-rotating hubs 64, 68. The lower ends of rods H8 and theshoes I I8 connected therewith are held in a proper radial position ineach gimbals assembly by gimbals spacer yokes I02, II4. These yokes areradially fixedly, slidably and pivotally related to their adjacent hubsand rotor shafts. This relation is accomplished in each case byproviding the shafts 88 and I0 with collar I20 secured'thereto, andhaving diametrically opposed slots I22 in which move shoes I24 thatcarry the pins I26, on eah pair of which is pivotally mounted a gimbalsspacer yoke I02, H4. The upper and lower gimbals spacer yokes, shoes,and blade rods of the upper and lower gimbals assembly rotate inopposite directions since they are attached to the counter-rotaryshafts. The upper ring H0 and track II2 of the upper ring gimbalsassembly,

to effect other functions to be explained hereafter,

it is advisable to include means for easily maintaining a collectivepitch position once it is set into the system. One method is the use ofa friction device at the fulcrum pin 82, such as fiber washerscompressed between the matching surfaces of the support arm 84 and slidebushing 18. The pressure may be adjusted to give the proper degree ofresistance to motion between these parts and thus leave the pilot freeto make other and more frequent adjustments on the control handle. Forthis purpose the bifurcations of arm 84 may be drawn together by bolt Iand wing nut I32.

In order to control the horizontal movement of helicopters through theair while they are being sustained at the same time by the liftdeveloped by rapidly rotatin blades, it has been found that cyclicpitching, or cyclically changing the angle of attack of each bladeduring its travel around the hub center, is necessary. For coaxialcounter-rotary machines it may be advisable to arrange the controlmechanism so that cyclic pitching can be impressed upon the blades ofboth rotors simultaneously. The phasing, or the positioning of the pointat which counter-rotating blades should reach their maximum or minimumangle of attack in their travel about the hub center to give the maximumor most efflcient total lateral thrust, is a debatable question but onewhich does not essentially affect this invention since, by properlypositioning the shoe ends of rods III; and the diametrically opposedslots I22 radially about the hubs 64 and 68, any phasin can be obtainedwith mechanisms described herein.

Figure 6 shows that by pushing the control handle I4 forward the bladesof both rotors are cyclically pitched in such a manner as to create anunbalanced total lift and, therefore, thrust in a forward direction.Figure 7 shows a similar condition in which a motion of the controlhandle 14 to the left results in a thrust to the left. The mechanics ofthe control, in fact, allow the control handle I4 to be displaced in anydirection away from vertical with a resulting motion of the craft inthat direction.

In Figure 3, assuming that the slide bushing I8 is stationary with thesupport arm 84 and with the case of transmission 44 because of thefriction of the washers 80, the control handle I4 may pivot fore and afton pin I34, and from side to side about the center line of rod I6journalled in the slide bushing I8. By swinging on the pin I34 handle I4carries rod I36 connected thereto fore and aft, which action in turnswings crank arm I38 on yoke pin 88. Since the arm I38 is connected bystrap I40 to the inner ring 94 of the lower gimbals assembly 92, anymotion of the control handle I4 in a forward or aft direction results ina tipping of the gimbals assembly 92 fore or aft. Since the rod I6 isattached to yoke 06 which, in turn, supports the lower gimbals assembly92 by straps I40 and I42, any right or left motion of the control handleI4 will tip the lower gimbals assembly 92 to the right or leftv Anymotion of the control handle I4 away from the vertical then results inthe lower gimbals assembly 92 tipping in the same direction, as will,also,-the upper gimbals assembly I08, since all tipping motions of thelower assembly are transmitted to it by rods I06. Both gimbalsassemblies 92, I08, since they are universally attached to theirrespective rotor shafts I0, 66, will likewise function at any rotorspeeds and maintain any tilted position so determined by the controlhandle 'I4. It is evident, then, that as the paired blades I2 arerotated by their hubs 64or 68, and since their roots I44 are turning ina fixed plane perpendicular to the axis of the coaxial shafts 66, III,the blades will changes their angles of attack about their pitching axisexactly as determined by the tilt or tipping of the ring tracks I00 andI I2, in which the rotary track shoes II8 are traveling, and to whichthe ends of the blade pitch control rods I I6 are attached. Therefore,the positioning of control handle I4 in any direction other thanparallel to the axis of the coaxial rotor shafts 66, I0, will impress acyclic changing of attack angles of all rotating blades I2 in a mannercreating greater lift in portions of their rotational sweep. Suchinduces a transverse thrust in the same direction as the handle has beenmoved and a subsequent motion of the craft in that direction. I Thecontrol handle I4, rods I6 and I36, and arm I38 constitute aparallelogram hinged at each corner, the motions of which are limited bythe slide bushing 18, through which the upper rods I6 may slide fore andaft slightly. Thus, cyclic pitching is not affected by any verticaldisplacement of the control handle I4 in adjusting collective pitch, andthe opposite is equally true because at any collective pitch position ofcontrol handle I4, cyclic pitch may be impressed as desired upon thesystem by appropriate movement of the same control handle I4.

In order for the pilot to perform all the-useful flight maneuversinherent in machines of the helicopter type. one additional controlprovision s necessary. While in flight, particularly so in the hoveringcondition, it is desirable that he be furnished means for facing thecraft in any direction by rotating it about its upright axis. In fact,since the craft is attached to, and suspended under, rotating elementswhich have been arranged in such a way as to transmit both the torqueand counter-torque of the engine to the surrounding air, it is necessaryto furnish means for easily adjusting or balancing these torques so thatthe craft itself will not constantly rotate in one direction or theother. The fact' that a greater resistance to rotation of one rotor ascompared with the resistance to rotate in the opposite direction of theother rotor will result in the craft itself rotating in this sameopposite direction is utilized in this invention to furnish directionalcontrol. The resistance to the rotation of either upper or lower rotoris accomplished by increasing the angle of attack, and thus, theaerodynamic drag of the blades of that rotor.

Referring to Figure 3, control handle I4 is shown free to rotate aboutits longitudinal axis in sleeve fitting I46 which also holds it inproper operating position. The rotary motion of handle 14 is translatedinto rotary motion of the outer ring 98 by means of the linkage systemcomprising arm I40 on the handle I4, arm I50 on the ring 98, and thepush-pull link I52 connected between said arms by ball and socketfittings I54. This system constitutes a second flexible parallelogrambetween the handle I4 and the lower imbals assembly 92, and operates thedirectional control system regardless of the position of the controlhandle as it is employed for its other control functions.

As has been noted above, the lower gimbals assembly 92 is composed ofthe inner ring 94 and track 96 which determine the blade angles of thelower rotor, and the outer ring 98 and track I00 which, by means of rodsI06, control the position of the upper gimbals assembly I08 and, thus,the blade angles of the upper rotor. inner ring 94 and track 96 are heldmotionless radially by straps I40 and I42 and pins and 90, which areattached to the yoke 86. The outer ring and track 98, I00 are free toboth rotate around and to slide up and down relative to the outside ofthe inner ring 94 and track 96, re-

The

7 stricted only by three or more pins I58 which are rigidly attached tothe inner ring 94, and which protrude through cam slots I58 cut in theouter ring 98. These cam slots I58 are cut in a helical pattern so thatany rotation of the outer ring 98 will result in its being raised orlowered relative to the inner ring 94. Since these rings respectivelydetermine the pitch angles of the upper and lower rotor blades, atwisting of the control handle 14 will thus introduce a differentialadjustment between blade angles of the two rotors.

As illustrated in Figure 8, the twisting of the control handle 14 aboutits longitudinal axis clockwise as viewed from. above will effect theincrease in pitch of the blades of the lower rotor as compared with thatof the blades in the upper rotor. Since the lower rotor is rotatingcounterclockwise, an increase in its blade pitch, and subsequentincrease in its resistance to rotation, will result in a reaction at thetransmission and, thus, within the suspended crafta reaction of a.similar magnitude and in the opposite direction.

Such reaction in this example will be in a clockwise direction, and thecraft then will rotate in the same direction that the control handle istwisted. Rotating the control handle counterclockwise will increase thepitch of the blades of the upper rotor, as compared with that of theblades of the lower rotor, and the craft will then .rotatecounter-clockwise.

For purposes of illustrating my invention more clearly, the followingmythical flight may be considered typical. Assume that the craft shownin Figure 1 is standing idle in a small cleared area, surrounded bybuildings or trees similar to those found in most urban areas, and thatthe pilot wishes to be transported to some other similar spot severalmiles distant. The machine has been checked for mechanical worthiness,and the fuel tank has been filled with the proper mixture of fuel andlubricant. The pilot may seat himself in the machine and fasten thesafety,

belt 24 and check the manual clutch control handle 50 to make sure thatthe clutch is disengaged. He or an assistant may then start the engine,and when it is warmed sufliciently to insure trouble free continuousoperation, the pilot may position control handle 14 in the manner shownin Figure 4. This will set all the blades at zero angle of attack andthereby reduce their frontal area and, hence, their aerodynamic drag tothe minimum while the rotors are being brought up to flying speed. Thepilot may next engage the manually operable clutch slowly whileincreasing his throttle setting as the engine power is being absorbedthrough the transmission and by the rotors. The clutch will be fullyengaged at some speed considerably under that of flying, and the pilotmay then concentrate on only the control handle 14 and his throttlesetting. When the rotors have reached full flying speed as determined byeither the pilot's skill or by a governing device set to maintain theengine at optimum speed, the pilot will gradually force the controlhandle upward to a position similar to tha shown in Figure 5.

The craft will then rise vertically at a rate of speed determined by thespeed with which the control handle has been moved, and by the amount ofpower being delivered by the engine to the rotors. As the craft leavesthe groundit may be apparent, that the rotor torque may not be balanced,and the craft may begin rotating about its upright axis until the pilotcorrects it directionally by twisting the control handle about itslongitudinal axis as described.

He may wish to continue in verticalflight until he'has risen above thesurrounding obstructions, in which case he will allow the control handleto remain as in Figure 5 and maintain direction by holding the handle atthe point where he has twisted it to balance out uneven rotor torques.

When the craft has reached the height of a few feet, the pilot may lowerthe control handle and at the same time reduce throttle until the crafthas ceased rising and the engine is supplying just enough power, and therotors are producing Justenough lift, to sustain the craft. It will benecessary in laterally moving air to make adjustments from time to timein his cyclic pitching to keep the machine from moving laterally withrespect to the ground. The pilot will counteract these lateraltendencies by moving the control handle as in Figures 6 and 7, or, moreprecisely, in the direction opposite from that in which the machine hasbegun to move. While thus hovering, he may wish to face the machine in anew direction, in which case he twists the control handle in thatdirection, returning it again to the balance position when the machinehas made the maneuver.

To continue the rise, he will again push the control handle upward andincrease engine power. To execute a forward turning climb, he will pushthe control handle forward, twist it in the direction of turn, andprobably reduce throttle somewhat as he gains forward speed.

This is due to the fact that it requires lesspower for the craft toclimb laterally than it does to climb at the same rate vertically. Whenthe turn is complete, he will twist the handle to its neutral position.When the climb is complete, he will reduce collective pitch and enginepower slightly and continue forward flight. As his forward velocityincreases he may return the control handle from its forward position,Figure 6, to nearly that shown in Figure 5. When he has reached hisdestination he will pull the control handle back toward himself,whereupon the craft will reduce its forward speed and will begin to losealtitude. When all forward motion has ceased, the handle is returned tonormal as shown in Figure 5, and moved down slightly as the throttle -isreduced. The craft will then settle vertically at a rate determined bythe collective pitch and throttle setting. As it nears the ground, thepilot will increase this setting by again moving the handle upward andmaking ground contact lightly with the same maneuver as'described abovefor hovering.

If, during the course of the trip and at some considerable altitude, theengine should fail, or for some other mechanical reason power shouldcease being transmitted through. the over-run clutch 55 in Figure 2, therotors will continue revolving, due to their inertia, fora short time,

which will give the pilot sufllcient opportunity to adjust the controlhandle from that of full power as in Figure 5, to a position at whichthe rotor blades are at a small angle of attack, and which will allowauto-rotation. In this instance the rotors of the craft will continue torevolve due to the aerodynamic forces acting skill, the operator maycontrol the craft in such I sible.

' craft to fall at a greater rate of speed during its early descent,thereby building up rotor speed and inertia which, just prior to groundcontact, he may utilize for furnishing the craft with enough verticallift to reduce its fall to zero.

This maneuver, termed flare-out, will then allow the machine and pilotto drop the remaining few feet to a very light landing.

Although I have shown and described certain specific embodiments of myinvention, I am fully aware that many modifications thereof are pos- Myinvention, therefore, is not to be restricted except insofar as isnecessitated by the prior art and by the spirit of the appended claims,as follows.

I claim: I

, 1. In a helicopter, a pair of co-axial shafts, a blade adapted forvariable lift and associated with each said shaft, means for moving saidshafts in counter-rotation, between the paths centric of said secondannular track, all of said 1 tracks being longitudinally movable of saidshaft axes, a spacer yoke between each the first-and second said tracksand the inner and outer of said co-axial shafts respectively, each yokehaving a pivotal coupling with the adjacent track and, at ninety degreestherefrom, a second pivotal coupling with the adjacent shaft, said lastmentioned coupling being movable longitudinally of the shaft, a pitchcontrol link revolubly coupled to each the first and second annulartracks and pivotally coupled to an adjacent blade to vary the lift ofthe blade upon longitudinal movement of the said tracks, said second andthird tracks having therebetween co-operable screw elements operableupon rotary motion of one track relative the other track to producerelative longitudinal movement of each other along a common axis, linkmeans coupled between said first and second annual tracks, a guidecrosshead secured for rotation to the outer of said coaxial shafts andslidably engaged with the links between said first and second tracks,means for simultaneously moving said second and third trackslongitudinally of said shaft axes whereby said blades have variable liftimpressed thereon, means for tilting one of said tracks relative saidshaft axes, and a single manually operable element associated with saidmeans for effecting longitudinal movement of said tracks and said meansfor tilting said tracks and operatively associated with said second andthird tracks to impart a torque to one to produce relative rotary motiontherebetween.

2. In a helicopter, a pair of coaxial shafts, a pair of blades adaptedfor variable lift and associated with each said shaft, means for movingsaid shafts in counter-rotation, between the paths of said blades andmovable with one of said blades a first annular track, outside the pathsof said blades a second annular track, a third annular 10 trackconcentric of said second track, all of said tracks being longitudinallymovable of said shaft axes, itch control link means revolubly coupled toeach the first and second annular tracks and pivotally coupled to anadjacent blade to vary the lift of said blade upon longitudinal movementof said tracks, a guide cross head secured for rotation to the outer ofsaid co-axial shafts and slidably engaged with the links between saidfirst and second tracks,-said second and third tracks being relativelymovable of each other along a common axis, means for effecting saidrelative movement between said second and third tracks, link meanscoupled between said first and third tracks, means for simultaneouslymoving said second and third tracks longitudinally of said shaft axeswhereby said blades have variable lift impressed thereon, means fortilting one of said tracks relative said shaft axes, and a singlemanually operable element associated with said means for effectinglongitudinal movement of said tracks and said tilting means and saidmeans for relatively moving said second and third tracks.

3. In a helicopter, a pair of coaxial shafts, a blade adapted forvariable lift and associated with each said shaft, means for moving saidshafts in counter-rotation, between the paths of said blades and movablewith one of said blades a first annular track, outside the paths of saidblades a second annular track, a third annular track concentric of saidsecond track, all of said tracks being longitudinally movable of saidshaft axes, a pitch control link revolubly coupled to each the first andsecond annular tracks and pivotally coupled to an adjacent blade to varythe lift of said blade upon longitudinal movement of said tracks, saidsecond and third tracks being relatively movable of each other along acommon axis, co-operable screw elements between said second and thirdtracks for eifecting said relative movement, and operable upon rotarymotion of one screw element with respect to the other, link meanscoupled between said first and third tracks, means for simultaneouslymoving said second and third tracks longitudinally of said shaft axeswhereby said blades have variable lift impressed thereon, means fortilting one of said tracks relative said shaft axes, and a singlemanually operable element associated with said means for effecting1ongitudinal movement of said tracks and said tilting means and saidmeans for relatively moving said second and third tracks.

4. In a helicopter, a pair of coaxial shafts, a blade adapted forvariable lift and associated with each said shaft, means for moving saidshafts in counter-rotation, between the paths of said blades and movablewith one of said blades a first annular track, outside the paths of saidblade a second annular track, a third annular track concentric of saidsecond track, all of said tracks being longitudinally movable of saidshaft axes, a pitch control link revolubly coupled to each the first andsecond annular tracks and pivotally coupled to an adjacent blade to varythe lift of said blade upon longitudinal movement of said between thefirst track and the inner of said coaxial shafts and between said secondtrack and 11 12 the outer of said coaxial shaft, a spacer yoke con-REFERENCES CITED pied for rotation with each shaft and mounted Thefollowing references are of m m the for slidabie movement therealong,said vote: each having a pivotal couple with its respective track thispatent at ninety degrees from its couple cl11th the shaft. 5 UNITEDSTATES PATENTS means for tilting one 01' said tra relative said shaftaxes, and a single manually operable eleg f ment associated with saidmeans for effecting 1 454's -;"""'v"" my 1928 longitudinal movement ofsaid tracks and said 1'912354 Pescm my 1933 tilting means and said meansfor relatively mov- 10 Cox Feb. 1946 and 2,410,533 Thomson o 5, 19462,427,936 Wales Sept. 23, 1947 Home! 2,448,073 Bendix Aug, a1, 1943-

